Hi everyone.
FYI - I spent the best part of a day pickiing brains to find out the proper facts on crankshaft model vs con rod model. I think a lot of us on here where under the impression that all the GX Series parts where retro fitt-able to the earlier model motors. Not quite true. Here it is in a nutshell.

CRANK SHAFTS
Gen 5.5, GX Series TiNi and MG66 TiNi coated cranks can be fitted to all Generations of GO engines - but you must use the right con rod.

CON RODS
The new Pro Con Rod (part # 21-0600) can only be fitted to the Gen 5.5, GX Series TiNi, and MG66 TiNi cranks. This rod is NOT compatible with any Gen 5 or earlier motor.
The Gen 5 and earlier motors require the older Pro Con Rod (part # 28-0600)
Reason being the differing crank pin sizes and internal diameters of the con rod big end bushings given by Massive Mods in his earlier post.

TiNi WRIST PINS
These are compatible with all the GO motors.

For those of you who were unaware, I hope this sheds some light.